A conventional method of zone transfer while a mobile unit is standing by is the one specified in “Personal Digital Cellular Telecommunication System RCR Standard”, RCR STD-27G published by ARIB (Association of Radio Wave Industry Business). In STD-27G, information for notifying about the current zone (hereinafter, reported information) is transmitted from the network to a mobile unit. FIG. 12 and FIG. 13 are diagrams showing the contents of the message of the conventional reported information specified in STD-27G.
The reported information contains a standby permission level (indicated by reference numeral 51) indicating the receiving level in the range of each radio zone/sector where a mobile station (mobile unit) can stand by, a standby deterioration level (indicated by reference numeral 52) indicating the receiving level at which it is determined that the mobile station standing by in each radio zone/sector has been deteriorated from the standby state, the perch channel number (indicated by reference numerals 53 and 54) providing the number of perch channel where the level is monitored in the determination of the current zone/sector, and the zone service information (indicated by reference numeral 55) for notifying the mobile station of the service condition in the current zone and the surrounding zones.
The operation of the conventional mobile unit will be explained here. FIG. 14 is a diagram for explaining the operation of measuring the surrounding level performed by the conventional mobile unit. When measuring the surrounding level, the mobile unit measures the receiving level a plurality of times (indicated by reference numeral 58) using all the surrounding perch channels notified in the reported information in the vacant slots associated with the reported information receiving slots (indicated by reference numeral 56) and the PCH receiving slot (indicated by reference numeral 57) of the local station, obtains an average of the measured values, and makes this average as a measurement value of the surrounding level.
FIG. 15 is a flowchart showing the sequence of the zone transfer operation performed by the conventional mobile unit. When performing the zone transfer operation, the mobile unit first measures the surrounding level using all the surrounding perch channels notified in the reported information (step S51) Then, the receiving level Lo of the local station slot is compared with the standby deterioration level Lth notified in the reported information (step S52). If the receiving level Lo of the local station slot is equal to or greater less than the standby deterioration level Lth, i.e. when the electric field is strong, the maximum max(Li) of the receiving level of the surrounding zone is compared with Lo+ΔL (step S53). ΔL in this equation can be determined as follows:ΔL=(Ltn−Lth)
If the maximum max(Li) of the receiving level of the surrounding zone is greater than Lo+ΔL in step S53, the standby channel select mode prevails, and the zone transfers to the surrounding perch channel of maximum level (step S54) If the maximum max(Li) of the receiving level of the surrounding zone is equal to or less than Lo+ΔL, on the other hand, the process in step S51 is carried out. If the receiving level Lo of the local station slot is less than the standby deterioration level Lth in step S52, i.e. when the electric field is weak, then the maximum max(Li) of the receiving level of the surrounding zone is compared with the standby permission level Ltn (step S55). If the maximum max(Li) is greater than Ltn, the process in step S54 is carried out, while if the maximum max(Li) is equal to or less than Ltn, the perch channel is scanned (step S56).
The conventional mobile unit adapted for packet communication also operates substantially in the same manner as the conventional mobile unit already above described. At the time of zone transfer in standby mode, however, the mobile unit transfers always to a zone adapted for packet communication where the packet communication service is available. Specifically, in step S54 of FIG. 15, the mobile unit transfers to the surrounding perch channel zone of maximum level adapted for packet communication.